General Morplioloyy 13 



section is made. Figure 7 (E) is a section through the sacculate part of 

 the kidney below tlie pericardium, and shows the lumen partly filled with 

 folds extending from the walls of the kidney. In fig. 8 (D) the section 

 shows the tubular portion of the kidney with a few folds extending from 

 the walls, also the large pulmonary vein at the right and the smaller renal 

 vein at the left. In figs. 9 and 10 (C, B) the same features are shown. In 

 fig. 11, near the lower part (A), the kidney is flattened and broadened 

 and the veins are smaller. All figures show the variation in the form of 

 the ridge in different portions of the kidney. The blood vessels (veins) lie 

 in close proximity to the tubular jiart of the kidney and pick up the blood 

 to be carried to the heart. 



The folds iu' the tubular portion of the kidney are metameric in their 

 presence and one section may pass through a portion of the kidney where 

 these are present, or it may pass between two sets of folds, as shown in 

 figs. 16 and 17 on plate 45, as well as on other plates. The ridge on the 

 kidney is mostly confined to members of the subfamily Helisomatinae 

 and offers a valuable criterion for the separation of this planorbid group. 

 The Planorbinae are mostly without this ridge. 



The ureter of the Planorbidae is described as straight, but no exit of 

 this nature was found in any species examined. All had a short ureter, 

 partly reflexed, as shown in the figures on the plates. 



[d] The Circulatory System 



The circulatory system of the Planorbidae is similar to that of the Lym- 

 naeidae and other Basommatophorous Pulmonata. The heart lies to the 

 right of the kidney (instead of to the left as in Lymnaea) and consists of 

 a ventricle and an auricle enclosed in an oval pericardium (plate 44, fig. 2, 

 PC, AU, VT). The aorta (AO) extends backward from the ventricle and 

 divides into two branches or arteries, the visceral artery which supplies 

 the digestive, genital, and other systems in the posterior part of the body, 

 and the cephalic artery which bends around and forward, carrying blood 

 to the organs in the head, foot, etc. These arteries branch frequently, 

 the branches becoming smaller, and terminate in contact with the venous 

 system either by small arterioles or through lacunae or blood sinuses which 

 connect the arterial system with the venous system. 



The venous system consists of blood vessels which convey the blood 

 back to the heart after circulation through the kidneys, lung, and other 

 parts of the system. A large branchial vein borders the kidney on the right 

 side, carrying blood to the auricle. A large renal vein also borders the left 

 side of the kidney, entering the saccular portion (see plate 44, fig. 2, AP, 

 RA). Venous sinuses are present and store the blood after circulation to be 

 carried to the renal and respiratory systems for purification. 



In the Planorbidae the blood (haemolymph) contains a substance 

 (haemoglobin) which gives it a pinkish or red color. This is an active 

 substance occurring abundantly in the respiratory system and differs from 

 the bluish blood (liaemocyanin) which occurs in most mollusks. Its pres- 

 ence in Planorbis is said to be associated with the poorly oxygenated hab- 

 itat occui^ied by most species of this group in stagnant pools or lakes and 

 ponds poorly supplied with oxygen. This statement, however, is open to 

 question, since many species of the Planorbidae live in habitats abundantly 



